The observations recorded in this paper were undertaken in consequence of certain spontaneous deflections having been noticed in the needles of the Electric Telegraph on the Midland Railway. The telegraph is constructed on the principle patented by Messrs. Wheatstone and Cooke, and the signals are made by deflecting a magnetic needle placed in a coil, to the right or left, by means of a galvanic battery. It was observed that when no signals were passing, and when the wires of the telegraph had simply connexion with the earth at the two termini, spontaneous deflections, differing in amount and direction, occasionally occurred. It was also observed in the four principal lines of telegraph which unite at Derby as a centre, two of which proceed in a northerly direction to Leeds and to Lincoln, and two in a southerly direction to Birmingham and to Rugby, that the relative deflections of the four instruments were such as to indicate that when the current of electricity, which produced the deflection, flowed from Rugby northwards towards Derby, it was also flowing northwards in all the other three; and likewise, that when it flowed southwards in one, it flowed southwards in all; the times of the deflections being simultaneous or nearly so. There appeared to be no regularity as to the hours, either during the day or night, at which these deflections occurred. Atmospheric electricity also affected the instruments, but in general only by sudden and violent effects during thunder storms, sometimes reversing the poles of the needles contained in the coils, and sometimes fusing the wire of the coil itself. But the effects first mentioned appeared to arise from a different cause; and from the great extent of line affected simultaneously by currents in the same direction, it appeared impossible they could arise from local atmospheric influences. On the night of Friday the 19th of March, there appeared a brilliant aurora, and during the whole time of its remaining visible, rapidly alternating deflections were exhibited in the telegraph instruments. The occurrence of these phenomena induced the author, with deflectometers of very delicate construction, to make a series of experiments, from which the following results were deduced. Wires insulated throughout, and wires having only one connexion with the earth, produced no deflection; and a complete circuit made by uniting both extremities of two wires, each forty-one miles long, but insulated throughout, produced no deflection. In every case, however, a deflection was obtained on a wire having both ends connected with the earth, which deflection was continually varying in amount and sometimes in direction.
V.—Note on the Geographical Distribution of Volcanos